Fast handover with reduced service interruption for high speed data channels in a wireless system

ABSTRACT

A method is provided for controlling a wireless communications system. The method comprises communicating data to a first base station, and selecting a switchover time for ceasing communications with the first base station and beginning communications with a second base station. The switchover time is based on channel conditions associated with the communications to the first and second base stations, such as SIR.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to telecommunications, and, more particularly, to wireless communications.

2. Description of the Related Art

In the field of wireless telecommunications, such as cellular telephony, a system typically includes a plurality of base stations distributed within an area to be serviced by the system. Various users within the area, fixed or mobile, may then access the system and, thus, other interconnected telecommunications systems, via one or more of the base stations. Typically, a mobile device maintains communications with the system as the mobile device passes through an area by communicating with one and then another base station, as the user moves. The mobile device may communicate with the closest base station, the base station with the strongest signal, the base station with a capacity sufficient to accept communications, etc.

Historically, the mobile device has been used for voice communications where the delivery of information is time critical. That is, if even relatively short segments of a conversation are delayed or lost, the meaning and understanding of the parties to the conversation may be substantially impaired. During the period when the mobile device is discontinuing communications with a first base station and beginning communications with a second base station, there is a distinct possibility that communications will be at least temporarily interrupted or delayed. Thus, for voice communications, a process known as soft hand off (SHO) was developed in the CDMA and UMTS systems to have multiple connections in the region of overlapped coverage in order to substantially enhance the likelihood that the conversation will continue unabated even during these transition periods.

Recently, the operation of mobile devices has been extended to the field of high speed data, such as might be employed when accessing the Internet or the World Wide Web. The exchange of high speed data, unlike voice communications, has historically not been time critical. That is, the transmission of data may be temporarily interrupted or delayed without affecting a receiver's ability to “understand” the data. Thus, temporary delays or interruptions during the transition period from one base station to another have been acceptable.

However, the use of high speed data connections has expanded to operations that are more time critical. For example, Voice over Internet Protocol (VoIP) is a process that involves digitizing voice signals, organizing the digitized voice signals into packets, and transmitting the packets over a high speed digital connection. A receiving party reassembles the packets and plays the packets to produce an audio communication. Thus, voice communications can be accomplished over a high speed data connection. If this process can be accomplished in real time, then a conversation may occur across the high speed digital connection. Where the high speed digital connection is being used for voice communications, then the transition periods become significant so as to avoid delaying or interrupting the conversation.

The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect of the instant invention, a method is provided for controlling a communications system. The method comprises communicating information to a first base station, and selecting a switchover time for beginning communications with a second base station. The switchover time is based on channel conditions associated with the communications to the first and second base stations.

In another aspect of the instant invention, a method is provided for controlling a communications system. The method comprises communicating information from a first base station, and selecting a switchover time for beginning communications from a second base station. The switchover time is based on channel conditions associated with the communications from the first and second base stations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:

FIG. 1A is a block diagram of a communications system, in accordance with one embodiment of the present invention;

FIG. 1B is a stylistic representation of a region in which the communications system of FIG. 1A may be employed;

FIG. 2 depicts a block diagram of one embodiment of a Base station and a mobile device used in the communications system of FIG. 1; and

FIG. 3 is a flow diagram illustrating the interoperation of the various components of the communications system of FIGS. 1 and 2.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

Turning now to the drawings, and specifically referring to FIG. 1A, a communications system 100 is illustrated, in accordance with one embodiment of the present invention. For illustrative purposes, the communications system 100 of FIG. 1A is a Universal Mobile Telephone System (UMTS), although it should be understood that the present invention may be applicable to other systems that support data and/or voice communication. The communications system 100 allows one or more mobile devices 120 to communicate with a data network 125, such as the Internet, and/or a public telephone system (PSTN) 160 through one or more base stations 130. The mobile device 120 may take the form of any of a variety of devices, including cellular phones, personal digital assistants (PDAs), laptop computers, digital pagers, wireless cards, and any other device capable of accessing the data network 125 and/or the PSTN 160 through the base station 130.

In one embodiment, a plurality of the base stations 130 may be coupled to a Radio Network Controller (RNC) 138 by one or more connections 139, such as T1/E1 lines or circuits, ATM virtual circuits, cables, optical digital subscriber lines (DSLs), and the like. Although one RNC 138 is illustrated, those skilled in the art will appreciate that a plurality of RNCs 138 may be utilized to interface with a large number of base stations 130. Generally, the RNC 138 operates to control and coordinate the base stations 130 to which it is connected. The RNC 138 of FIG. 1 generally provides replication, communications, runtime, and system management services, and, as discussed below in more detail below, may be involved in coordinating the transition of a mobile device 120 during transitions between the base stations 130.

As is illustrated in FIG. 1B, a region 170 to be serviced by the system 100 is separated into a plurality of regions or cells, each being associated with a separate base station 130. Typically, each cell has a plurality of adjacent neighboring cells. For example, the cell 175 has six neighboring cells 176-181 such that a mobile device 120 entering the cell 175 may travel from one of the neighboring cells 176-181. Thus, as the mobile device 120 enters the cell 175 from any of the neighboring cells 176-181, the mobile device may need to transition from communicating with the cell 175 to communicating with the neighboring cell 176-181 that it is entering.

Returning to FIG. 1A, the RNC 138 is also coupled to a Core Network (CN) 165 via a connection 145, which may take on any of a variety of forms, such as T1/E1 lines or circuits, ATM virtual circuits, cables, optical digital subscriber lines (DSLs), and the like. Generally the CN 165 operates as an interface to the data network 125 and/or to the public telephone system (PSTN) 160. The CN 165 performs a variety of functions and operations, such as user authentication, however, a detailed description of the structure and operation of the CN 165 is not necessary to an understanding and appreciation of the instant invention. Accordingly, to avoid unnecessarily obfuscating the instant invention, further details of the CN 165 are not presented herein.

Thus, those skilled in the art will appreciate that the communications system 100 enables the mobile devices 120 to communicate with the data network 125 and/or the PSTN 160. It should be understood, however, that the configuration of the communications system 100 of FIG. 1A is exemplary in nature, and that fewer or additional components may be employed in other embodiments of the communications system 100 without departing from the spirit and scope of the instant invention.

Unless specifically stated otherwise, or as is apparent from the discussion, terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical, electronic quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system's memories or registers or other such information storage, transmission or display devices.

Referring now to FIG. 2, a block diagram of one embodiment of a functional structure associated with an exemplary base station 130 and mobile device 120 is shown. The base station 130 includes an interface unit 200, a controller 210, an antenna 215 and a plurality of channels: such as a shared channel 220, a data channel 230, and a control channel 240. The interface unit 200, in the illustrated embodiment, controls the flow of information between the base station 130 and the RNC 138 (see FIG. 1A). The controller 210 generally operates to control both the transmission and reception of data and control signals over the antenna 215 and the plurality of channels 220, 230, 240 and to communicate at least portions of the received information to the RNC 138 via the interface unit 200.

The mobile device 120 shares certain functional attributes with the base station 130. For example, the mobile device 120 includes a controller 250, an antenna 255 and a plurality of channels: such as a shared channel 260, a data channel 270, and a control channel 280. The controller 250 generally operates to control both the transmission and reception of data and control signals over the antenna 255 and the plurality of channels 260, 270, 280.

Normally, the channels 260, 270, 280 in the mobile device 120 communicate with the corresponding channels 220, 230, 240 in the base station 130. Under the operation of the controllers 210, 250, the channels 220, 260; 230, 270; 240, 280 are used to effect a controlled scheduling of communications from the mobile device 120 to the base station 130.

Turning now to FIG. 3, a flow diagram illustrating the interoperation of the various components of the system 100 is shown. In the flow diagram of FIG. 3, it is assumed that a high speed data transmission is underway with respect to the mobile device 120 such that the mobile device 120 is communicating with base station A, but will be transitioning to base station B. Initially, the mobile device 120 is within the cell associated with base station A and is approaching or entering the cell associated with the base station B.

FIG. 3 stylistically represents a handover procedure for a high speed data channel, generally depicting a messaging process that may be used to switch over the high speed data channel from a serving cell to a target cell. Generally, the actual switchover begins when the RNC 138 sends a Radio Link Reconfiguration Commit messages to the serving Node B to stop the scheduled transmission at a defined “activation time.” The mobile device 120 begins “listening” to scheduling information from the target cell at the activation time after sending the “Physical Channel Reconfiguration Complete” messages. A significant factor in reducing VoIP service interruption is the setting of the “activation time” for the switchover from the serving cell to the target cell. Historically, the “activation time” is set at the very early stage of Radio Link Reconfiguration Commit messages and is executed at the time when the mobile device 120 sends the “Physical Channel Reconfiguration Complete.” In some prior art systems, the setting time and the execution time may be separated by a significant amount of time (e.g., from hundreds of milliseconds to seconds). The “activation time” is determined based on the process time of the signaling messages (e.g., Iur, Iub and UU). During this interval, a variety of factors could negatively impact the radio channel conditions. For example, the mobile device 120 may move away from the serving cell and the radio link quality could deteriorate significantly such that data could no longer be delivered. It could also happen that mobile device 120 may be unable to decode the scheduling information and missed the data. Thus, VoIP service to the mobile device 120 may be interrupted for a significant amount of time. In one embodiment of the instant invention, the activation time for the switchover process is carefully selected based on current conditions of the radio link between the mobile device 120 and the serving and target base stations.

In a first embodiment of the instant invention, as shown in FIG. 3, the serving RNC 138-1 determines that there is a need to switchover from high speed communications between the mobile device 120 and a source base station 103-1 to high speed communications between the mobile device 120 and a target base station 130-2. The serving RNC 138-1 prepares an RNSAP (Radio Link Reconfiguration Prepare) message, which is transmitted to a drift RNC 138-2 at 300.

In the illustrated embodiment, the source and target cells are controlled by different base stations 130-1, 130-2. The drift RNC 138-2 requests the source base station 130-1 to perform a synchronized radio link reconfiguration using an NBAP (Node B Application Part) message “Radio Link Reconfiguration Prepare” at 302. The source base station 130-1 returns an NBAP message “Radio Link Reconfiguration Ready” at 304.

The drift RNC 138-2 requests the target base station 130-2 to perform a synchronized radio link reconfiguration using the NBAP message “Radio Link Reconfiguration Prepare” at 306. The target base station 130-2 returns the NBAP message “Radio Link Reconfiguration Ready” at 308. The drift RNC 138-2 returns an RNSAP message “Radio Link Reconfiguration Ready” to the serving RNC 138-2 at 310. The drift RNC 138-2 initiates set-up of a new Iub Data Transport Bearers using ALCAP (Access Link Control Application Protocol) protocol at 312. This request contains an AAL2 (ATM Adaptation Layer type 2) Binding Identity to bind the Iub Data Transport Bearer to the high speed data channel.

The serving RNC 138-1 initiates set-up of a new Iur Data Transport bearer using ALCAP protocol at 314. This request contains the AAL2 Binding Identity to bind the Iur Data Transport Bearer to the high speed data channel. The high speed data channel transport bearer to the target base station 130-2 is established. The serving RNC 138-1 proceeds by transmitting the RNSAP message “Radio Link Reconfiguration Commit” to the drift RNC 138-2 at 316. The serving RNC 138-1 selected the activation time in the form of a CFN (Connection Frame Number).

The drift RNC 138-2 transmits the NBAP message “Radio Link Reconfiguration Commit” to the source base station 130-1 including the activation time at 318. Similarly, the drift RNC 138-2 also transmits the NBAP message “Radio Link Reconfiguration Commit” to the target base station 130-2 including the activation time at 320. At the indicated activation time, the source base station 130-1 stops transmitting and the target base station 130-2 starts transmitting on the high speed data channel to the mobile device 120.

The serving RNC 138-1 also transmits an RRC (Radio Resource Control) message “Physical Channel Reconfiguration” to the mobile device 120 at 322. At the indicated activation time the mobile device 120 stops receiving high speed data in the source cell and starts receiving high speed data in the target cell. The mobile device 120 returns an RRC message “Physical Channel Reconfiguration Complete” to the serving RNC 138-1 at 324. The drift RNC 138-2 initiates release of the old Iub Data Transport bearer using ALCAP protocol at 326. Similarly, the serving RNC 138-1 initiates release of the old Iur Data Transport bearer using ALCAP protocol at 328.

To reduce or minimize VoIP service interruption during handover, the “activation time” needs to be properly selected. The “activation time” is determined by the processing time of the signaling procedure. If the switchover is set very late and the radio channel condition of the serving cell has deteriorated, the VoIP service will have an open window period of no service. If the activation time is set too early and the radio channel condition of the target cell relative to the specific user is not good, the VoIP service will have an interruption too. The issue of the switchover timing is the unawareness of the radio channel condition when RNC 138-1 sets the activation time. It requires L-3 signaling to communicate between the mobile device 120 and RNC in order to identify the best timing of switchover. Nevertheless, the L-3 signaling delay is large to affect the effectiveness of the switchover timing identification. Thus, the criteria of minimizing the VoIP service interruption during the handover are to estimate the required signaling processing time correctly, to reduce the signaling time, and to switch over to the target cell at the right time.

Reducing the time required for signaling during switchover can improve the switchover process. The RNC 138 uses feedback of the radio channel conditions, such as SIR (Signal to Interference Ratio) from the base stations 130 in the active set or mobile reported best cell measurement, to trigger the switchover and to estimate a desired switchover time. However, the accuracy of the estimation deteriorates as the prediction interval becomes larger. In particular, radio channel conditions may changes dramatically in hundreds of milliseconds to seconds. By reducing the interval, the switchover process may be substantially improved.

The base station 130 periodically reports SIR measurements to the RNC 138. In one embodiment, the average SIR is calculated over an 80 ms interval and reported to the RNC 138. The average SIR measurement may be used as a reference of the radio channel condition of each leg during the switchover. The average uplink SIR measurements are a reciprocal of the long-term average of the CQI reports for the downlink radio link. In one embodiment of the instant invention, an algorithm used to calculate “activation time” is as follows,

$\left\{ \begin{matrix} {{SIR}_{Serving} < {Threshold}_{Serving}} \\ {{SIR}_{Target} > {Threshold}_{target}} \end{matrix}\quad \right.$ where SIR_(Serving) and SIR_(Target) are the SIR measurements of the serving cell and the target cell, respectively, and Threshold_(Serving) and Threshold_(target) are the thresholds of the serving cell and target cell, respectively. The algorithm is designed based on link imbalance during the switchover when the uplink inner loop power control would speed up the migration of the radio link to the leg with good channel condition. Thus, the activation time setting could be improved by setting the threshold properly in accordance with the signaling process delay.

The general signaling procedures of switching the high speed data channel from the serving cell to the target cell are performed in series. Since Iub Radio Link Reconfiguration Commit message requires no acknowledgement, the delay may be reduced by sending Iub Radio Link Reconfiguration messages to the serving and target cells as well as UU RRC Physical Layer Reconfiguration message to the mobile device 120 at about the same time, or at least with some overlap. This would remove significant delays and introduce minimum process delay only.

With the concurrent Iub and UU signaling message to commit the switchover of the radio links at the activation time, the processing delay of the UU RRC Physical Channel Reconfiguration message is another factor in the delay. The current performance procedure for the Physical Channel Reconfiguration (see section 13.5.2 of TS25.331) requires the mobile device 120 to execute it within 80 ms after receiving the UU signaling message and update the L1 configuration at the beginning of the next TTI. The DCCH RABs supported in the TS34.108 are 1.7 kbps, 3.4 kbps, and 13.6 kbps with TTI being 80, 40, and 10 ms, respectively. The most common test procedure is the 3.4 kbps DCCH with 40 ms TTI. If the 3.4 kbps DCCH were used to carry the Physical Channel Reconfiguration messages for the high speed data channel switchover, the total delay would exceed 120 ms (80 ms processing+40 ms TTI DCCH reception). Moreover, the 80 ms execution delay is also considered by the low signaling RAB for communication of the primitive between higher layer protocols and physical layers at the mobile device 120. Using the 13.6 kbps RAB with 10 ms TTI would reduce the execution time and processing time. It will reduce the interruption of the VoIP service during switchover.

One significant issue of the high speed data channel switchover is to synchronize the buffers at the base stations 130-1 and 130-2 since both Iub links for the serving cell and target cell are established before the Radio Link Reconfiguration Commit commands. To reduce the service interruption, the RNC 138 can send the VoIP data to both of the base stations 130-1 and 130-2 after the Iub data link of the target base station 130-2 is established.

The target base station 130-2 will not schedule any transmission before the “activation time” and has no idea of the scheduled information at the source base station 130-1. Since other channels (e.g., UL DPCH) are operating in the soft-handover mode, the target base station 130-2 may decode the Ack/Nack and CQI information before the “activation time” although that they intend to send to the source base station 130-1. Due to the link imbalance during the switchover, the target cell would decode the Ack/Nack and CQI for the serving cell incorrectly with erasure in the beginning and gradually move into correct decoding before the “activation time”. The target cell could decide to drop a VoIP frame every T_(erasure) interval if the Ack/Nack and CQI are erased. The T_(erasure) interval is a parameter to be optimized for minimizing the buffer occupancy at the target cell. If the Ack is decoded correctly at the target cell before the activation time, the target cell could determine to drop the next VoIP packet. This procedure is an uncoordinated VoIP counting for the high speed data channel when the VoIP packet has been sent to both the serving and target cells during the switchover before the activation time. This will avoid any data gap for the sequence delivery of the VoIP packet and minimize the buffer occupancy at the target cell. The proposed algorithm is called “Pseudo-Synchronization” because its behavior is similar to the buffer synchronization.

Those skilled in the art will appreciate that the various system layers, routines, or modules illustrated in the various embodiments herein may be executable control units (such as the controllers 210, 250 (see FIG. 2)). The controllers 210, 250 may include a microprocessor, a microcontroller, a digital signal processor, a processor card (including one or more microprocessors or controllers), or other control or computing devices. The storage devices referred to in this discussion may include one or more machine-readable storage media for storing data and instructions. The storage media may include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy, removable disks; other magnetic media including tape; and optical media such as compact disks (CDs) or digital video disks (DVDs). Instructions that make up the various software layers, routines, or modules in the various systems may be stored in respective storage devices. The instructions when executed by the controllers 210, 250 cause the corresponding system to perform programmed acts.

The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. Consequently, the method, system and portions thereof and of the described method and system may be implemented in different locations, such as the wireless unit, the base station, a base station controller and/or mobile switching center. Moreover, processing circuitry required to implement and use the described system may be implemented in application specific integrated circuits, software-driven processing circuitry, firmware, programmable logic devices, hardware, discrete components or arrangements of the above components as would be understood by one of ordinary skill in the art with the benefit of this disclosure. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below. 

1. A method for controlling a handover in a communications system that provides a plurality of channels comprising at least one high-speed data channel that supports real-time, high-speed data communications, the method comprising: communicating data to a first base station that is configured to transmit the data to a mobile unit over said at least one high-speed data channel during real-time, high speed communication; receiving feedback of channel conditions from the first base station and at least one second base station, wherein the feedback indicates channel conditions determined using measurements of at least one of the plurality of channels other than said at least one high-speed data channel; and initiating a soft handover such that the mobile unit is in concurrent communication with the first and second base stations using channels other than said at least one high-speed data channel selecting, subsequent to initiating the soft handover and at a radio network controller that is communicatively coupled to the first and second base stations, a switchover time for a hard handover of said at least one high speed data channel, the switchover time indicating a time at which the first base station stops transmitting data to the mobile unit over said at least one high-speed data channel and the second base station starts transmitting data to the mobile unit over said at least one high-speed data channel, and wherein said switchover time is selected to reduce service interruption during real-time, high speed communication by selecting the based on channel conditions associated with the communications to the first and second base stations; transmitting a first message from the radio network controller to the first and second base stations, the first message indicating the switchover time; stopping communication between the mobile device and the first base station at the switchover time; and starting communication between the mobile device and the second base station at the switchover time.
 2. A method, as set forth in claim 1, wherein selecting the switchover time for beginning communications with the second base station further comprises selecting the switchover time for beginning communications with the second base station wherein said switchover time is based on SIR associated with the communications to the first and second base stations.
 3. A method, as set forth in claim 2, wherein selecting the switchover time for beginning communications with the second base station wherein said switchover time is based on SIR associated with the communications to the first and second base stations further comprises selecting the switchover time for beginning communications with the second base station at a time when SIR associated with the communications to the first base station falls below a preselected threshold.
 4. A method, as set forth in claim 2, wherein selecting the switchover time for beginning communications with the second base station wherein said switchover time is based on SIR associated with the communications to the first and second base stations further comprises selecting the switchover time for beginning communications with the second base station at a time when SIR associated with the communications to the second base station rises above a preselected threshold.
 5. A method, as set forth in claim 2, wherein selecting the switchover time for beginning communications with the second base station wherein said switchover time is based on SIR associated with the communications to the first and second base stations further comprises selecting the switchover time for beginning communications with the second base station at a time when SIR associated with the communications to the first base station falls below a preselected threshold and SIR associated with the communications to the second base station rises above a preselected threshold.
 6. A method, as set forth in claim 1, further comprising sending a reconfiguration message to the first and second base stations to allow communications to begin with the second base station.
 7. A method, as set forth in claim 6, wherein sending the reconfiguration message to the first and second base stations to allow communications to begin with the second base station further comprises sending the reconfiguration message to the first and second base stations at about the same time to allow communications to begin with the second base station.
 8. A method, as set forth in claim 6, wherein sending the reconfiguration message to the first and second base stations to allow communications to begin with the second base station further comprises sending the reconfiguration message to the first and second base stations during overlapping time intervals to allow communications to begin with the second base station.
 9. A method, as set forth in claim 6, wherein sending the reconfiguration message to the first and second base stations to allow communications to begin with the second base station further comprises allowing communications with the first base station to cease.
 10. A method, as set forth in claim 1, wherein communicating data to the first base station further comprises, communicating data to the first base station over a high speed data channel.
 11. A method, as set forth in claim 1, further comprising communicating data to both the first and second base stations during a first period of time after the switchover time is selected and before an actual switchover occurs.
 12. A method, as set forth in claim 1, further comprising storing the communicated data in a buffer in the second base station during the first period of time.
 13. A method, as set forth in claim 12, further comprising estimating at the second base station the data provided to the first base station during the first period of time.
 14. A method, as set forth in claim 13, further comprising clearing data from the second buffer that has been estimated as being provided to the first base station during the first period of time. 